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The advantage of MagSafe 2 over the original MagSafe is simply its slimness: It’s easier to fit the new, slimmer port into very thin Mac laptops.
The MacBook Air uses a 45W MagSafe 2 adapter; the MacBook Pro with Retina display uses an 85W MagSafe 2 adapter. If history is a guide, you can mix and match those MagSafe 2 adapters, but the lower-powered one will take longer to charge the MacBook Pro. (The 85W one can’t charge a MacBook Air any faster, either.)
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MagSafe 2 (left) and last year's model.
And you don’t need to throw away your old MagSafe adapters yet. The tiny, $10 MagSafe to MagSafe 2 Converter lets you use your old power brick with your new laptop.